Wind turbine blades for rotors of wind turbines currently have a length of about 40 meters or more, sometimes about 90 or 100 meters. The wind turbine blades need to be very stiff and, thus, usually constitute a shell structure comprising a metal or composite material. Parts of cars or aeroplanes need to fulfil similar physical requirements concerning the composite materials. Such composite materials commonly consist of a plastic material that is reinforced with fibres such as glass fibres, carbon fibres or others. A growing number of such products are manufactured by vacuum assisted resin transfer moulding processes.
In the VARTM a stack of fibres or a fibre stack is placed in a mould. The mould is closed by a second mould part or a plastic liner is placed over the mould. Vacuum is applied to the fibre filled hollow mould structure by a vacuum pump connected to one opening of the mould. A resin is then infused from a resin container, optionally a resin mixing facility, into the fibre stack. The provided resin fills out the space between the fibres and is finally cured in the mould. In this way a rigid reinforced composite structure having a specific shape is manufactured. The resin flows into the mould cavity through inlet holes distributed over the mould surface. In conventional VARTM processes resin flow channels in the mould inner surfaces or in the core material surfaces are used to permit for a faster flow of resin to certain areas of the mould system. The use of extra resin channels in the mould surfaces and in the core material surfaces enhances the penetration of resin from both sides of a laminate into the fibre stack, to get a fast wetting of the total fibre stack. However, the extra resin channels take up extra amounts of resin, which make the composite parts manufactured heavier and more expensive.
Alternatively, it is known in the prior art to place a special resin flow enhancing member, also called flow sheet (e.g. a fibre filled tube or sheet member), between the plastic liner (e.g. a vacuum film or vacuum foil) and the fibre stack to be wetted by the resin. When the resin is sucked into the mould cavity it flows fast through the resin flow enhancing member, whereas the fibre material is denser and exhibits a higher resistance to the resin flow. Partly forced by vacuum and partly by capillary forces, the resin is distributed into the fibre material. As the resin flows over an area, the foremost area of the resin in movement is called the resin flow front.
During a VARTM process the highest vacuum is found in the air filled space in front of the resin flow front. The static vacuum declines behind the resin flow front. Due to the differences in static pressures, the resin flows fast through the resin flow enhancing member, mainly passing over the fibre material to be wetted. Thus, a correct wetting of all fibres inside the mould cannot be guaranteed, especially in fibre stacks having a substantial thickness and/or length.